a very simple and easily used expression. For pipes of circular section m = d/4, where d is the diameter:—
u0 = √ [ (gcτd / 4ζ l) {(p02 − p12) / p02} ];
(7b)
or approximately
u0 = (1.1319 − 0.7264 p1/p0) √ (gcτd / 4ζ l).
(7c)
§ 91. Coefficient of Friction for Air.—A discussion by Professor Unwin of the experiments by Culley and Sabine on the rate of transmission of light carriers through pneumatic tubes, in which there is steady flow of air not sensibly affected by any resistances other than surface friction, furnished the value ζ = .007. The pipes were lead pipes, slightly moist, 21⁄4 in. (0.187 ft.) in diameter, and in lengths of 2000 to nearly 6000 ft.
In some experiments on the flow of air through cast-iron pipes A. Arson found the coefficient of friction to vary with the velocity and diameter of the pipe. Putting
ζ = α/v + β,
(8)